We set ours at 10C, our thstat is web enabled so it can E-mail us and our house sitter if it's below setpoint. Normally we shut off the water and water heater completely, this winter our daughter will spend a weekend or two at our house so I will set the water heater on vacation mode and shut off the water. She will turn on the water and w/h and return them to the settings we left them at when she leaves.
It take a few hours to go from 10 to zero C so the house sitter has reaction time to call the HVAC guy before a hard freeze happens